In recent years, portable electronic terminals such as a notebook-sized personal computer, a cellular phone, and a personal digital assistant (PDA) are remarkably spread. As a secondary battery used as a power source for these portable electronic terminals, for example, a lithium ion secondary battery is often used. The portable electronic terminals are required to have a comfortable portability, and therefore reductions in size, thickness, and weight, and an increase in performance are rapidly promoted. As a result, the portable electronic terminals are used in a variety of situations. Like the portable electronic terminals, the secondary battery is also required to reduce the size, thickness, and weight, and increase the performance.
For improving performance of the secondary battery, improvement of an electrode, an electrolytic solution, and other members of the battery is investigated. Of these, the negative electrode is usually produced by dispersing or dissolving a polymer used as a binder in a solvent to prepare a liquid composition, mixing a negative electrode active material with the liquid composition to obtain a slurry composition, applying the slurry composition onto a current collector, and then drying the applied slurry composition. It has hitherto been attempted to achieve a higher-performance secondary battery by selecting a binder suitable for the negative electrode produced by the aforementioned method.
For example, in Patent Literature 1, an attempt has been made to improve the cycle property of a lithium ion secondary battery by using, as a binder, two types of carboxy-modified styrene-butadiene copolymer latexes each having a different glass transition temperature.